Expansion cards are known, for example those called PCI, used to connect and/or manage an external apparatus or device, for example a monitor, a peripheral unit, etc., on an electronic apparatus such as a personal computer.
In order to conform to the standard of size, such expansion cards have to have a unified thickness to allow the male connector present on the edge of the card to be inserted into the female connector, or slot, located on the mother card of the electronic apparatus.
Various connection standards exist which have respective unified values: an example of this is the PCI standard, according to which the thickness of the connector on the card must be equal to 1.57±mm 0.2 mm; other examples, among the many others, are the SODIMM standard (standard thickness 1.0 mm), the miniPC standard (thickness 1.0 mm) and the DIMM standard (1.27 mm).
Cards used at present have a problem when they are required to manage a large number of digital signals at high speed, with recourse to controlled impedance paths. This situation occurs, for example, when cards and systems are designed for the field of High Performance Computing (HPC). In this case, in fact, it is necessary to use particular structures so as to have transmission lines with controlled impedance: the paths are located between mass islands, in a kind of sandwich structure, or in other, similar configurations. These structures allow to control and maintain the impedance constant along the electric track. The quicker the signal rise time is, and the longer the path, the more fundamental it is to have a good control of the impedance values on the line.
The applications mentioned above entail the presence of a large number of signals which, arranged in groups, move from one point to another, generating a sort of communication network between the computing nodes. This large number of signals must therefore be managed using transmission lines with controlled impedance and, if we want to arrange said rapid digital signals correctly, it is necessary to arrange a lot of layers on the printed circuit of the PCI card.
The need to have a large number of layers clashes with the need to respect a card thickness which satisfies the constraint of size, as imposed by the standard for inserting the card into the slot located on the mother card.
Some production processes are well-known, which allow to increase the number of layers without infringing the aforesaid requirements of thickness, technological limits and impedance values. Among these production processes it is known to use additional processes with sequential coating (and laser drilling), or with the introduction of particular materials for the dielectric sub-strates, which determine a lesser value of the dielectric constant and hence the possibility of using thinner sub-strates.
These processes allow to reduce the thickness of some of the sub-strates present in the card, and in this way it is possible to add auxiliary signal layers.
However, there are some designs for cards wherein these methods cannot be used or they do not have a great real utility.
One example is the case where a large number of signals has to travel inside the printed circuit, following parallel paths, in the same direction.
A possible approach to manage these signals is to provide a local coupling of cards using card-to-card connectors or to introduce recent card-to-card welding technologies. However, there may be numerous disadvantages deriving from the use of these technologies:                increase in the overall height of the system, for example due to the need to have two cards, one above the other, with a certain distance between them;        additional assembly processes to allow to make the coupling system, for example assembly of the connectors or card-to-card welding;        introduction of additional materials, for example connectors, etc.        
These features can make the relative production processes not advantageous in the case of some designs for cards.
Document U.S. Pat. No. 6,421,250B1 concerns a memory module to be associated with an electronic apparatus that comprises a printed circuit including, on one side, two protrusions on the surfaces of which connection feet are made for association with corresponding connection elements of the electronic apparatus. The purpose of the protrusions is to increase the number of feet in contact between the memory module and the electronic apparatus without increasing the size of the memory module itself.
However, this solution not only requires dedicated and specific connection slots, but also starts from a premise that is totally different with respect to the problem to which the present invention is intended to supply a solution.
In fact, while U.S.'250 achieves a specific connection geometry to keep the size of the memory module constant, in the present invention it is desired to obtain an increase, for example, in the processing capacity or in the functions of the expansion card, by increasing the number of the layers that make up at least part of the printed circuit, without modifying the geometry of the standard connections of the usual electronic apparatuses.
U.S. Pat. No. 4,740,414 also describes a solution wherein a printed circuit has, on one side, a specific connection part on which a plurality of connection contacts are made. In this case too, however, the purpose is to increase the number of connection points, keeping the size of the printed circuit unchanged; the purpose is not to increase the calculation capacity of the expansion card without modifying the standard connection part connecting to a slot of an electronic apparatus.
Purpose of the invention is to achieve a multilayer expansion card for an electronic apparatus which will allow to increase the number of planes or layers usable for managing the signals, in any case ensuring that the connection standards are respected (for example the PCI standard mentioned above), both in terms of size and also in terms of mechanical profile and arrangement of the contact areas.
Another purpose is to achieve an expansion card without having recourse to using particular dielectrics or additional independent cards and additional assembly processes.
The present Applicant has devised, tested and embodied this invention to obtain these purposes, to overcome the shortcomings mentioned above, and to obtain other advantages.